Structural factors that mediate scleral stiffness.

PURPOSE: The intent of this study was to correlate measures of structurally relevant biochemical constituents with tensile mechanical behavior in porcine and human posterior sclera.
METHODS: Posterior scleral strips 6 x 25 mm were harvested from 13 young porcine and 10 aged human eyes and stored frozen at -20 degrees C. Mechanical hysteresis from 10 consecutive load cycles to a peak stress of 1.0 MPa was recorded via a custom-built soft tissue tester. In a parallel study, tissue adjacent to the mechanical test specimens was apportioned for each of five assays measuring: total collagen content, nonenzymatic cross-link density, elastin content, glycosaminoglycan content, and water content.
RESULTS: The average porcine scleral modulus at 1% strain was 75% less than that measured for human tissue (0.65 +/- 0.53 MPa versus 2.60 +/- 2.13 MPa, respectively; P < 0.05). However, the average strain energy absorbed per loading cycle was similar (6.09 +/- 2.54 kJ/m(3) vs. 5.96 +/- 2.69 kJ/m(3) for porcine and human sclera respectively; P > 0.05). Aged human sclera had relatively high fluorescence due to nonenzymatic cross-link density (2200 +/- 368 vs. 842 +/- 342; P < 0.05) and low hydroxyproline content (0.79 +/- 0.17 microL/mL/g versus 1.21 +/- 0.09 microL/mL/g; P < 0.05) while other measured biochemical factors were statistically similar (P > 0.05).
CONCLUSIONS: Aged human tissue had superior mechanical stiffness despite reduced collagen content, partially because of the accumulation of nonenzymatic cross-links. Differences in collagen content and cross-link density either had no effect or offsetting effects on the ability of the tissues to absorb strain energy.